Novikov Self-Consistency Principle: Why Time Travel Need Not Create Paradoxes

The Novikov self-consistency principle is a proposal in the physics of time travel developed by Russian physicist Igor Novikov in the mid-1980s. It states that if an event would give rise to a paradox or otherwise alter the past, the probability of that event occurring is zero. Equivalently, the only solutions to the laws of physics that can occur locally are those that are globally self-consistent. The principle was formalized in a 1990 paper by Friedman, Morris, Novikov, Echeverria, Klinkhammer, Thorne, and Yurtsever titled "Cauchy problem in spacetimes with closed timelike curves." Rather than forbidding time travel, the principle allows closed timelike curves while ensuring a time traveler's actions become part of history in ways that avoid grandfather paradox contradictions, through physical constraint rather than choice. The best-known demonstration is the 1991 billiard-ball analysis of Polchinski's paradox: a ball fired into a wormhole at an angle that would seem to knock its earlier self off course instead receives a glancing blow that nudges it precisely onto the consistent trajectory. Multiple, even infinitely many, self-consistent solutions can exist for the same initial conditions. The principle has strong implications for determinism and free will: a time traveler cannot change the past, only enact what already happened. Physicist J. Craig Wheeler has noted this is less alien than it sounds, since physics already constrains action every day, as gravity prevents unaided flight. The principle underlies why a causal loop or bootstrap paradox can be self-consistent rather than contradictory. See Closed Timelike Curve: Time Loops in General Relativity.